A motor for driving a compressor that constitutes an on-vehicle air conditioner is controlled in its operation by an inverter system.
As shown in FIG. 2, a conventional inverter system 1 operates at a low voltage of about 5 V converted by a non-isolated DC-DC converter 4 from a high voltage of 100 V or more supplied from a high voltage power source 3 for driving a motor 2. The inverter system 1 communicates with a host ECU 6 via a communication interface 5 for controlling an on-vehicle air conditioner or the like. Other electrical equipment systems of a vehicle including the host ECU 6 or the like for the inverter system 1 operate at a voltage of 12 V or 24 V supplied from an on-vehicle battery power source 7. The high voltage for driving the motor 2 being applied to the other electrical equipment systems for some reasons may lead to a failure, and thus the inverter system 1 and the other electrical equipment systems can communicate with each other while being isolated by an isolated communication connector 8 such as a photocoupler (for example, see Patent Documents 1 and 2).    Patent Document 1: Japanese Patent No. 3152154    Patent Document 2: Japanese Patent No. 3351330